1. Field of the Invention
The present invention relates generally to Power over Ethernet (PoE) and, more particularly, to a system and method for measuring a cable resistance in a PoE application.
2. Introduction
The IEEE 802.3af and 802.3at PoE specifications provide a framework for delivery of power from power sourcing equipment (PSE) to a powered device (PD) over Ethernet cabling. In this framework, various PDs can be deployed such as voice over IP (VoIP) phones, wireless LAN access points, network cameras, computing devices, etc.
In the PoE process, a valid device detection is first performed. This detection process identifies whether or not it is connected to a valid device to ensure that power is not applied to non-PoE capable devices. After a valid PD is discovered, the PSE can optionally perform a Layer 1 power classification. For example, in the IEEE 802.3af standard, the classification step identifies a power classification of the PD from the various power classes of 15.4 W, 7.0 W, and 4.0 W. In various PoE implementations, a Layer 2 power classification process can be initiated to reclassify the power class or implement some form of dynamic classification.
After the classification process is complete, the PSE would allocate power to the port. In a typical usage scenario, the PSE has a fixed power budget that can easily be oversubscribed by the connected PDs. Management of such a fixed power budget can therefore dictate that lower priority PDs would not receive a power allocation from the PSE.
In this fixed power budget environment, it is important for the PSE to accurately determine a power budget for the various powered ports. One factor that can impact the power budget for the particular port is the physical characteristics (e.g., resistance) of the cable connecting the PSE and the PD. The more information that is available regarding the physical characteristics of the cable, the greater the accuracy in the power budget assigned to the individual ports. What is needed therefore is a mechanism for accurately identifying the cable characteristics in a PoE application.